Front and rear wheel drive vehicles are generally known, in which both front and rear wheel pairs of the vehicle are driven to improve running through performance of the vehicle. Conventionally, these front and rear wheel drive vehicles are classified into part time four wheel drive type vehicles and full time four wheel drive type vehicles in accordance with the type of driven wheel. In the part time four wheel drive type, the vehicle is switched between four wheel drive and two wheel drive in response to the road conditions and driving conditions, and when the vehicle is in a bad road condition or on a steep slope, the vehicle runs with the front and rear wheels driven, and when the vehicle is in a good road condition, the vehicle runs with two wheels driven to improve the fuel economy.
However, in this part time four wheel drive type, since the driving force is divided from the engine as a power plant into the front wheels and the rear wheels, a complicated and expensive power transmission device is required.
In view of the foregoing drawbacks of the prior art, the applicants have repeated considerable research and development works to provide a hybrid-type front and rear wheel drive vehicle, wherein a front wheel pair and a rear wheel pair are driven with different power units, i.e. one of the front and rear wheel pairs is driven with an engine and the other one of the front and rear wheel pairs is driven with a motor having the same output with the engine, and wherein a traction and a slip and further fuel consumption are controlled in such a manner that control signals outputted from the controller control the actuation, stop or the drive of the engine and the motor.
Specifically, the vehicle speed is detected and the rear wheel slip ratio is obtained based on the average rotational speed of the right and left and front and rear wheels and the vehicle speed, and subsequently the control mode of the vehicle is determined based on the shift position, on/off of the accelerator, the opening degree of the accelerator pedal and the driving condition of the vehicle.
For example, when the accelerator pedal is on and the advance acceleration is outputted from an acceleration sensor, an advance driving mode is determined, and when the rearward acceleration is outputted from the acceleration sensor, a rearward driving mode is determined.
ECU composed of a microcomputer comprises a fixed memory such as RAM (Random Access Memory) or ROM (Read Only Memory), and driving force distribution control programs for determining the driving force distribution ratio of the engine and the motor and at least a program considering fuel consumption and a program considering running through performance are stored in the fixed memory so that a suitable program is selected in accordance with the driving mode.
In this event, consideration is made to the above driving force distribution control programs so that the driving condition is determined from the accelerator opening degree, the vehicle speed and the like, and based on this driving condition the driving force command values of the engine and the motor are determined with respect to the demand driving force required for the present driving condition, and subsequently when the driving force command values are outputted to the engine and the motor, these values are modified to values associated with the actual driving force and thereafter outputted to the output control sections of the engine and the motor.
However, as shown in FIG. 9, the engine and the motor are different in change of the actual driving force ratio after the driving force command value is outputted and before reaching to the maximum output, and as a result, the engine and the motor are considerably different in terms of the time constant up to the maximum transitional period. For this reason, a torque slippage arises due to a temporal decrement of the rotating torque of the motor for example when the regeneration mode is carried out to charge the battery under the command of the battery remaining amount sensor during the front and rear wheel driven mode at which the engine and the motor are driven simultaneously, and when the target driving force of the engine increases with respect to the target driving force of the motor for the purpose of fuel-saving drive and the driving ratio of the engine increases as the result.
As mentioned above, according to the hybrid-type front and rear wheel drive vehicle independently controlling the engine and the motor, a shock arises due to torque slippage or excess torque when one driving force command value of the engine or the motor decreases with respect to the other driving force of the motor or the engine and a large torque gap arises.
In order to prevent the shock, the time constant of the motor associated with the transitional period from the minimum target driving force to the maximum target driving force may be always as large as that of the engine, and the command value of the motor may be filtered with a delayed filter having the same level time constant with the engine, viz. a delayed filtering process may be carried out so that the time constant of the motor is substantially the same as that of the engine. However, this leads to a loss of the excellent feeling of the motor with excellent response, and as the result, the advantages of the hybrid-type front and rear wheel drive vehicle are lost.
Accordingly, the object of the present invention is to solve the drawbacks, such as a shock derived from torque slippage or excess torque resulting from differences of the time constant between the engine and the motor, in the front and rear wheel drive vehicle wherein one of front and rear wheel pairs is driven with the engine and the other one of the front and rear wheel pairs is driven with the motor.